1. Field of the Invention
The present invention relates to CMOS image sensors. In particular, the invention relates to one or more isolation wells disposed between the substrate and a photodiode in a pixel of a CMOS image sensor to induce a field that repels photo generated charge from drifting into the photodiode.
2. Description of Related Art
CMOS image sensors are pushing to ever lower operating voltages and to ever smaller pixel pitches. They are also being targeted at more markets in which cheap illumination is an advantage. This generally means that the illumination wavelength is moving farther from the blue and into the near-infrared. These trends are leading to ever grater levels of electronic crosstalk in CMOS image sensors. Electronic crosstalk in this sense refers to the diffusion of photocharge that is generated outside (usually beneath) the depleted photosite into neighboring photosites or collection nodes. The result is that the charge generated by the light that is incident on a given pixel migrates to neighboring pixels and is interpreted as signal charge. This crosstalk leads to degradations in performance measures such as image spatial resolution (sharp features in the image become washed out) and color separation (red light enters a red pixel but shows up as signal in a blue pixel).
Crosstalk is also an issue for pixel architectures in which each photosite is accompanied by (an) adjacent sense/storage node(s). The purpose of the storage node is to store charge that was collected on a photosite during the previous integration period so as to hold that charge until a readout operation can take place. During the storage time it is undesirable to have photocharge diffuse onto the storage node. This leads to a xe2x80x9ccorruptedxe2x80x9d level on the storage node which is subsequently read out. The same mechanism that leads to photosite crosstalk also produces storage node crosstalk.
It is an object to the present invention to overcome limitations in the prior art. It is a further object of the present invention to provide a sensor that isolates a photodiode from photo charge generated in nearby silicon.
These and other objects are achieved in an embodiment of a sensor formed in a substrate of a first conductivity type in a first concentration that includes CMOS circuitry to control the sensor, a first well of the first conductivity type in a second concentration formed in the substrate, and a photodiode region of a second conductivity type formed in the first well. The second concentration is greater than the first concentration.
These and other objects are also achieved in an embodiment of a sensor formed in a substrate of a first conductivity type that includes CMOS circuitry to control the sensor, a first well of a second conductivity type formed in the substrate, a second well of the first conductivity type formed in the first well, and a photodiode region of the second conductivity type formed in the second well.
These and other objects are achieved in a method embodiment of the invention that includes a step of applying a first potential to a first well formed in a substrate of a first conductivity type where the substrate further includes CMOS sensor control circuitry formed in the substrate and the first well is formed to have a second conductivity type. The method further includes a step of applying a second potential to a second well of the first conductivity type that is formed in the first well where a photodiode region of the second conductivity type is formed in the second well. The first and second potentials induce a field between the first and second wells that repels photo generated charge from drifting from the first well into the second well.